Data Flat Vacancy Rates in Zurich

Does anyone here know how they calculate .07%?

“Nur 0,07 Prozent der Wohnungen stehen in der Stadt leer.”

https://www.20min.ch/story/diese-gem…h-399886865498

Tax declarations? If it's not your main residence, either you rent it out or not... What about semi-vacant apartments, where the owner/tenant is not living there for extended period of time traveling etc, dunno